During the last two decades many microprocessor controlled knee joints (MPK) appeared on the market.The newly developed C-Leg 4 aims to provide improved standing function while in the same time offering technology of knee´s previous version (C-Leg 3). Since added functional benefit of a knee joint cannot be anticipated just due to the fact that the knee is controlled by the microprocessor, the aim of this post-market study is to evaluate the safety, functional mobility and satisfaction of the microprocessor controlled prosthetic knee components C-Leg 3 and C-Leg 4 in comparison to mechanical knee joints. Additional goal is to improve the methodological quality of research conducted in the field.